The invention relates to an optical head for scanning a surface, which head is provided with a source supplying a radiation beam, an input grating having a periodic grating structure for coupling the radiation beam as a deflected beam into a waveguide, and an output grating for coupling radiation out of the waveguide and for directing said radiation in the form of a scanning beam onto the surface. Such an optical head can be used in apparatuses for inspecting surfaces in which, for example, contours in a surface are scanned. An example of such an apparatus is a scanning microscope. The head can also be used in a device for scanning recording layers in an optical record carrier. The head can be used for writing and reading the recording layer.
An optical scanning head of the type described in the opening paragraph is known from European Patent Application no. 0,357,780. In this optical head the radiation from the source is coupled into the waveguide via a circular input grating and is subsequently focused on a record carrier via an output grating. The use of a waveguide in this so-called planar head provides the possibility of giving it a very compact form. The waveguide increases the cross-section of the radiation beam from the radiation source to the cross-section of the output grating so that a good quality of the focus spot formed by the scanning beam on the surface to be scanned is obtained. If no use is made of a planar optical system, the desired cross-section of the radiation beam must be realised by making use of the divergence of the radiation beam or by widening the beam. In the first-mentioned case the optical head will be long because the divergence of most laser diodes which are used as radiation sources in optical heads is small. In the second case an extra component is required because a beam expander has at least one lens. When using a planar optical system, the conventional lenses may in principle be dispensed with so that the optical head becomes smaller and lighter.
In the optical head according to the above-mentioned European Patent Application deviations which may occur in the radiation beam from the source are not taken into account. The laser diode which is conventionally used as a radiation source in optical scanning devices supplies a radiation beam having a number of deficiencies. In the first place the cross-section of the radiation beam is not circular. A circular input grating will therefore not be irradiated throughout the surface. The radiation in the waveguide under the input grating will partly leak away due to the presence of the grating. The non-irradiated part of the grating has no function during coupling in, but it does increase the loss of radiation in the waveguide. In the second place the wavefront of the radiation beam from the source is generally astigmatic. Astigmatism deteriorates coupling in and coupling out of the radiation; moreover, it has a detrimental influence on the quality of the focus formed by the scanning beam. In the third place the wavelength of the laser diode may vary as a result of variations in temperature of and the current through the laser diode. The operation of gratings is greatly dependent on the wavelength. The known input grating will therefore no longer couple radiation into the waveguide in the case of a small variation of the wavelength.